Re: Dry Plate Speed & Shelflife

From: "D. Mark Andrews" <mark@dragonbones.com>
Subject: RE: Dry Plate Speed & Shelflife
Date: Tue, 01 Mar 2005 12:41:14 -0800

> The vintage dry plates I see on eBay I believe have an ISO of 25-50.

Those are made way after wet collodion process was made obsolete. The
very early silver gelatin emulsions were many many stops slower than
those numbers. I cannot make a color blind plate emulsion of daylight
speed comparable to double digit ASA without using refinements made by
King, Eder, and great many other amateur photographers of
1880s. Loosely speaking, ASA speed of 25-50 by today's standard is a
1920s technology.

> Bottled Silver Emulsions:
> Before learning the wet plate process I toyed with liquid emulsions on
> glass. I recall the ISO being somewhere north of 500--much faster than
> traditional wet or dry plate.

I think you are confused between different ratings of ISO emulsion
speeds. Speed for b&w negative emulsion is measured by 0.8/E where E
is the exposure necessary to make density of 0.1 above base plus fog
at normal contrast (average gradient of 0.6) in lux seconds. Speed for
b&w printing emulsion is measured by 1000/E where E is the exposure
necessary to make reflection density of 0.6 above fog in lux seconds.
These numbers are obtained at different conditions and they are not
directly comparable.

Commercial multigrade enlarging papers are in the neighborhood of ISO
P500. But when they are used in camera, practical EI range is single
digit, and they still give way too much contrast by todays standard.

In early era of silver gelatin, the negatives were customarily contact
printed onto POP paper, platinum paper, and other printing materials,
and therefore the negatives were made higher contrast than today's
standard.

--
Ryuji Suzuki
"Well, believing is all right, just don't let the wrong people know
what it's all about." (Bob Dylan, Need a Woman, 1982)